org-table: Fix orgtbl-self-insert-command on white spaces

* lisp/org-table.el (orgtbl-self-insert-command): Properly overwrite
white spaces when there is room for it. This is on par with what
`org-self-insert-command' does in Org tables.
Reported-by: Alex <agrambot@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/107112>